1. Field of the Invention
The present invention relates to an optical fiber for transmitting light, a method of making an optical fiber preform, and a method of making an optical fiber.
2. Related Background Art
In light transmissions using an optical fiber, transmission loss such as Rayleigh scattering loss caused by Rayleigh scattering within the optical fiber, structural asymmetry loss caused by disturbance in structures within the optical fiber, and the like becomes problematic.
These kinds of transmission loss are greatly influenced by the tension applied to the optical fiber when making the optical fiber by drawing an optical fiber preform upon heating. Namely, if the tension applied to the optical fiber is too low or too high with respect to the tension range considered favorable at the time of drawing, then the Rayleigh scattering loss, structural asymmetry loss, and the like within the optical fiber may increase. More specifically, the structural asymmetry loss increases at a lower tension. At a higher tension, on the other hand, both the Rayleigh scattering loss and structural asymmetry loss increase. Such a tension at the time of drawing also influences transmission characteristics of the optical fiber other than its transmission loss, its structures, its mechanical strength, and the like.
The tension applied to the optical fiber at the time of drawing usually changes with time during when the optical fiber preform is being drawn upon heating. Consequently, if the optical fiber preform is drawn as it is, the tension applied to the optical fiber preform may vary greatly over the whole length thereof, thereby making it difficult to make a long optical fiber yielding a low transmission loss. Therefore, tension control for keeping the tension within a favorable tension range is necessary in the optical fiber drawing step.
The above-mentioned favorable tension range at the time of drawing an optical fiber may vary depending on the structure and material of an optical fiber preform, its specific drawing condition, and the like. Here, if the tension range permissible as a drawing condition for yielding a favorable optical fiber is narrow, then it becomes very hard to carry out tension control at a sufficient accuracy over the whole length of the optical fiber preform.
For example, in an optical fiber (optical fiber preform) having a core made of pure SiO2 (pure silica), its core region yields a viscosity higher than that in its cladding region doped with F or the like (see, for example, xe2x80x9cHanawa et al., the Transactions of the Institute of Electronics, Information and Communication Engineers, 1989/3, Vol. J72-C-I, No. 3, pp. 167-176xe2x80x9d). Therefore, at the time of drawing the optical fiber preform, the stress occurring within the optical fiber is concentrated into the core, thus causing transmission loss to increase. For restraining the transmission loss from increasing due to the stress concentration into the core in such a case, severe tension control is necessary at a high accuracy, or it may become problematic in that tension control cannot be carried out in such a manner as to sufficiently lower the transmission loss, and so forth.
Also, xe2x80x9cSakaguchi, the Transactions of the Institute of Electronics, Information and Communication Engineers, 2000/1, Vol. J83-C, No. 1, pp. 30-36xe2x80x9d, discloses that annealing an optical fiber after drawing reduces the Rayleigh scattering within the optical fiber. Namely, the Rayleigh scattering intensity within glass is not constantly fixed by materials thereof, but depends on a fictive temperature Tf which is a virtual temperature indicative of the randomness in the state of arrangement of atoms within glass. Specifically, the Rayleigh scattering intensity increases as the fictive temperature Tf within glass is higher (randomness is greater).
In this regard, when drawing an optical fiber preform upon heating, a heating furnace is installed downstream a drawing furnace and is heated such that the drawn optical fiber attains a temperature within a predetermined temperature range when passing through the heating furnace. As a consequence, the heating by use of the heating furnace prevents the drawn optical fiber from cooling drastically, whereby the optical fiber is annealed. Here, due to the structural relaxation of glass caused by rearrangement of atoms, the fictive temperature Tf within the optical fiber decreases, whereby the Rayleigh scattering intensity within the optical fiber is suppressed.
However, the inventor has found that, even when such a manufacturing method yielding an effect of lowering the Rayleigh scattering loss is used, if the tension at the time of drawing the optical fiber preform is not within the favorable tension range, the structural asymmetry loss will increase due to the stress concentration into the core, and so forth, whereby the transmission loss may not be reduced as a whole.
In order to overcome the foregoing problems, it is an object of the present invention to provide an optical fiber, a method of making an optical fiber preform, and a method of making an optical fiber which facilitate the tension control at the time of drawing.
For achieving such a problem, the optical fiber in accordance with the present invention comprises a core region; and a cladding region, provided at an outer periphery of the core region, having one or a plurality of cladding layers doped with fluorine which lowers a refractive index, wherein the outermost cladding layer of the one or plurality of cladding layers is configured such that fluorine successively lowers the doping amount thereof in an outer peripheral part including an outer periphery thereof to a predetermined doping amount which is the minimum doping amount of fluorine within the layer.
In the above-mentioned optical fiber, among the cladding layers formed as being doped with F (fluorine), the outermost cladding layer is configured so as to yield such an F doping amount distribution that the doping amount of F gradually decreases from the inner side to the outer side within the outer peripheral part (the outer periphery and its vicinity) in the outermost cladding layer. Here, viscosity becomes higher in the outer peripheral part of the outermost cladding layer in which the doping amount of F is smaller, whereby the stress applied to the inside of the optical fiber is dispersed into the outer peripheral part of the outermost cladding layer, which suppresses the stress concentration into the core. Also, this stress dispersion makes it possible to widen the favorable tension range permissible at the time of drawing the optical fiber.
As a consequence, the optical fiber in accordance with the present invention becomes an optical fiber having a configuration which simplifies the tension control at the time of drawing. At the same time, the increase in transmission loss and the deterioration in transmission characteristics are prevented from occurring due to the excessive stress concentration into the core and the like, which realizes an optical fiber having stable transmission characteristics over the whole length thereof.
Since the region reducing the doping amount of F is the outer peripheral part of the outermost cladding layer, the F doping amount distribution has no influence over the light transmitted through the core region and the cladding region in the vicinity thereof. Therefore, while favorably keeping transmission characteristics and the like of the optical fiber, it can facilitate the tension control, or reduce the transmission loss thereby.
The method of making an optical fiber preform in accordance with the present invention comprises: (1) a synthesizing step of depositing a glass fine particle onto an outer periphery of a core preform including at least a core region, so as to synthesize a glass fine particle layer to become the outermost cladding layer in one or a plurality of layers of cladding in a cladding region disposed at an outer periphery of the core region; (2) a dehydrating step of dehydrating the synthesized glass fine particle layer upon heating; and (3) a consolidating step of consolidating the dehydrated glass fine particle layer upon heating so as to form the outermost cladding layer, thus forming an optical fiber preform having the core region and the cladding region having the one or plurality of layers of cladding; wherein (4), before consolidating the glass fine particle layer upon heating, doping the glass fine particle layer with fluorine and eliminating a part of doped fluorine from an outer peripheral part including an outer periphery thereof.
Drawing the optical fiber preform obtained by such a method of making an optical fiber preform makes it possible to yield an optical fiber which is doped with or rid of F such that the doping amount of F within the outer peripheral part in the outermost cladding layer in the cladding layers of the cladding region successively decreases to a predetermined doping amount which is the minimum doping amount of F within the layer.
Alternatively, the method of making an optical fiber preform in accordance with the present invention comprises: (1) a synthesizing step of depositing a glass fine particle onto an outer periphery of a core preform including at least a core region, so as to synthesize a glass fine particle layer to become the outermost cladding layer in one or a plurality of layers of cladding in a cladding region disposed at an outer periphery of the core region; (2) a dehydrating step of dehydrating the synthesized glass fine particle layer upon heating; and (3) a consolidating step of consolidating the dehydrated glass fine particle layer upon heating so as to form the outermost cladding layer, thus forming an optical fiber preform having the core region and the cladding region having the one or plurality of layers of cladding; wherein (4) the glass fine particle layer is synthesized in the synthesizing step such that the glass fine particle layer is doped with fluorine by using a material gas containing fluorine while the material gas containing fluorine is adjusted such that the doping amount of fluorine successively decreases within an outer peripheral part including an outer periphery thereof.
Drawing the optical fiber preform obtained by such a method of making an optical fiber preform similarly makes it possible to yield an optical fiber which is doped with F such that the doping amount of F within the outer peripheral part in the outermost cladding layer successively decreases to a predetermined doping amount which is the minimum doping amount of F within the layer.
The method of making an optical fiber in accordance with the present invention comprises preparing an optical fiber preform comprising a core region, and a cladding region, provided at an outer periphery of the core region, having one or a plurality of cladding layers doped with fluorine which lowers a refractive index, in which the outermost cladding layer of the one or plurality of cladding layers is configured such that fluorine successively lowers the doping amount thereof in an outer peripheral part including an outer periphery thereof to a predetermined doping amount which is the minimum doping amount of fluorine within the layer; and, when drawing the optical fiber preform upon heating, drawing the optical fiber preform at a tension within the range of 0.05 to 0.20 N.
Since the optical fiber preform (optical fiber) has such a configuration that the stress is dispersed into the outer peripheral part of the outer cladding layer, whereas the tension control is carried out such that the tension at the time of drawing is held within the favorable tension range of 0.05 to 0.20 N, an optical fiber having favorable transmission characteristics over the whole length thereof can be obtained.
Alternatively, the method of making an optical fiber in accordance with the present invention comprises preparing an optical fiber preform comprising a core region, and a cladding region, provided at an outer periphery of the core region, having one or a plurality of cladding layers doped with fluorine which lowers a refractive index, in which the outermost cladding layer of the one or plurality of cladding layers is configured such that fluorine successively lowers the doping amount thereof in an outer peripheral part including an outer periphery thereof to a predetermined doping amount which is the minimum doping amount of fluorine within the layer; and, when drawing the optical fiber preform upon heating, causing a heating furnace disposed downstream a drawing furnace to heat the optical fiber drawn by the drawing furnace to a temperature within a predetermined temperature range.
Thus, while the structure mentioned above suppresses the stress concentration and reduces the transmission loss, annealing the optical fiber by using the heating furnace disposed downstream the drawing furnace upon heating lowers the fictive temperature Tf within the optical fiber, which makes it possible to reduce the Rayleigh scattering loss.
When a resin coating section for coating the drawn optical fiber with a resin exists in the above-mentioned methods of making an optical fiber, it is preferred that the heating furnace disposed downstream the drawing furnace be located between the drawing furnace and the resin coating section.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.